ANSWER: B

Rationale:

Bioavailability (F) is defined as the fraction of an administered dose that reaches the systemic circulation in an unchanged form and is therefore available to exert pharmacological effects. For an intravenous dose, bioavailability is by definition 100% (F = 1.0) because the drug is delivered directly into the circulation. For oral and other extravascular routes, bioavailability is reduced by incomplete absorption, gut wall metabolism, and hepatic first-pass metabolism. It is expressed as F = AUC(oral) / AUC(IV) when doses are equal, or corrected for dose when they differ. Option A describes rate of absorption (related to Tmax) rather than the extent of bioavailability. Option C describes distribution, a separate pharmacokinetic process. Option D describes plasma protein binding, which influences distribution and free drug fraction but is distinct from bioavailability. Option E describes the time to reach half-maximal concentration, which relates to absorption rate but not bioavailability as a concept.

ANSWER: B

Rationale:

The primary mechanism of absorption for most lipid-soluble, low-molecular-weight drugs is passive transcellular diffusion — movement across the lipid bilayer of enterocyte membranes driven solely by the concentration gradient from the intestinal lumen into the cell and then into the portal circulation. This process is non-saturable, requires no energy, and is directly proportional to the drug's lipophilicity (logP) and the concentration gradient. Option A describes active transport, which requires energy and carrier proteins and is used for nutrients such as amino acids and glucose; some drugs (e.g., levodopa, certain cephalosporins) exploit active transport mechanisms. Option C describes paracellular diffusion through tight junctions, which is the primary route for small hydrophilic molecules and ions but is limited for most drugs by the impermeability of tight junctions. Option D describes endocytosis/pinocytosis, which is relevant for very large molecules such as vitamin B12 (via intrinsic factor) and some biologics but not for typical small-molecule drugs. Option E describes facilitated diffusion, which is carrier-mediated but not energy-dependent; it applies to specific nutrients and some drugs but is not the primary mechanism for lipid-soluble small molecules.

ANSWER: C

Rationale:

If oral bioavailability is 20% (F = 0.20), then only 20% of the oral dose reaches systemic circulation due to first-pass hepatic (and possibly intestinal) metabolism. Intravenous administration entirely bypasses first-pass metabolism, delivering 100% of the dose directly into the systemic circulation. With equal doses, the IV AUC will therefore be 1/F = 1/0.20 = 5 times greater than the oral AUC. This relationship is the basis for dose conversion between routes: if a patient requires 100 mg IV of a drug with 20% oral bioavailability and is being converted to oral therapy, the equivalent oral dose is 100/0.20 = 500 mg. Option A is incorrect — it inverts the relationship; it is the oral dose, not the IV dose, that has the lower AUC due to first-pass loss. Option B is incorrect — hepatic metabolic capacity operates on the fraction of drug reaching the liver via the portal vein after oral absorption; IV drug reaches the liver via the arterial circulation after systemic distribution, and critically, first-pass extraction occurs only with portal delivery. Option D describes a mathematically incorrect relationship. Option E is incorrect — while Vd and half-life determine the shape of the concentration-time curve, the ratio of IV to oral AUC for equal doses is determined directly and solely by bioavailability: AUC(IV)/AUC(oral) = 1/F.

ANSWER: B

Rationale:

Volume of distribution is a proportionality constant relating the total amount of drug in the body to the plasma drug concentration: Vd = Dose / C(plasma). It does not represent a real anatomical volume but rather an apparent volume that would be required if the drug were uniformly distributed at the same concentration as measured in plasma. A Vd of 350 L in a 70 kg adult greatly exceeds total body water (approximately 42 L) and plasma volume (approximately 3–4 L), indicating that the vast majority of drug has left the plasma and distributed into peripheral tissues — binding to muscle, fat, or intracellular proteins. Drugs with very large Vd (>100 L) include chloroquine (~200–800 L), amiodarone (~5000 L), and digoxin (~500 L), all of which have extensive tissue binding. This also means plasma concentrations are very low relative to the total drug in the body, which has implications for dialyzability — drugs with large Vd are not effectively removed by hemodialysis because very little drug resides in the plasma compartment. Option A is incorrect — drugs confined to plasma have small Vd (approximately 3–8 L); a Vd of 350 L indicates the opposite. Option C is incorrect — interstitial fluid distribution alone gives a Vd of approximately 15 L. Option D is incorrect — Vd is a distribution parameter and provides no direct information about the route of elimination. Option E is incorrect — water-soluble drugs typically have small Vd, not 350 L; a Vd of this magnitude implies lipophilicity and intracellular sequestration.

ANSWER: C

Rationale:

Phase I reactions are functionalization reactions that introduce or expose polar functional groups (hydroxyl, amino, carboxyl, thiol) on the drug molecule through oxidation, reduction, or hydrolysis. The most clinically important Phase I reactions are oxidations catalyzed by the cytochrome P450 (CYP) superfamily of enzymes in the hepatic endoplasmic reticulum. Phase I reactions increase the polarity of the drug and often generate a reactive intermediate that serves as a substrate for subsequent Phase II conjugation. Crucially, Phase I metabolites may be pharmacologically active (e.g., codeine morphine via CYP2D6), toxic (e.g., acetaminophen NAPQI via CYP2E1), or inactive. Options A, B, D, and E all describe Phase II reactions: glucuronidation (A) is catalyzed by UDP-glucuronosyltransferases (UGTs); sulfation (B) is catalyzed by sulfotransferases (SULTs); acetylation (D) is catalyzed by N-acetyltransferases (NAT1/NAT2) and is subject to the slow/fast acetylator polymorphism; glutathione conjugation (E) is catalyzed by glutathione S-transferases (GSTs) and is a critical detoxification pathway for reactive Phase I metabolites including NAPQI.

ANSWER: B

Rationale:

CYP inhibitors reduce the metabolic activity of the target CYP enzyme by competitive, non-competitive, or mechanism-based (irreversible) inhibition. When a CYP3A4 substrate is co-administered with a potent CYP3A4 inhibitor, the inhibitor reduces or eliminates the enzyme's ability to metabolize the substrate. This decreases the substrate's hepatic (and intestinal) clearance, increases its bioavailability if given orally, and raises its plasma concentrations — potentially into the toxic range. Clinically important CYP3A4 inhibitors include azole antifungals (ketoconazole, itraconazole, voriconazole), macrolide antibiotics (clarithromycin, erythromycin), HIV protease inhibitors (ritonavir), and grapefruit juice components (furanocoumarins). Option A describes the effect of CYP induction (e.g., rifampicin, carbamazepine, phenytoin), which increases enzyme activity and reduces substrate plasma levels. Option C is incorrect — CYP inhibitors do not induce CYP activity; induction is a separate mechanism requiring increased enzyme synthesis via nuclear receptor activation. Option D is incorrect — CYP inhibition affects all substrates, not only prodrugs; the distinction between prodrug and active drug is relevant to the direction of clinical consequences but does not exempt active drugs from the interaction. Option E is incorrect — CYP inhibition is a hepatic/intestinal metabolic interaction, not a renal transporter interaction; renal tubular secretion inhibition is a separate mechanism involving transporters such as OCT2 and MATE1.

ANSWER: C

Rationale:

This question tests the Henderson-Hasselbalch principle applied to renal drug excretion. For a weakly acidic drug (e.g., aspirin, pKa ~3.5), urinary pH determines the ionization state in the tubular lumen. In alkaline urine (pH 7.5–8.0), the drug exists predominantly in its ionized (deprotonated, negatively charged) form — the ion-trap phenomenon. Ionized molecules cannot passively diffuse across the lipid bilayer of the tubular epithelium back into the peritubular circulation, so reabsorption is minimized and the drug is excreted in the urine. Conversely, in acidic urine, the drug is predominantly unionized and lipid-soluble, allowing passive reabsorption. Alkaline urine thus increases the renal clearance of weak acids — a principle exploited clinically in salicylate poisoning (urinary alkalinization with sodium bicarbonate increases salicylate excretion). The question asks about reabsorption in alkaline urine, and the correct answer is that alkaline urine reduces reabsorption by ion trapping Option C correctly identifies this mechanism. Option A describes active secretion, a separate process that is pH-independent. Option B describes passive reabsorption in favorable (acidic) urine conditions for a weak acid — the opposite of what alkaline urine produces. Option D describes SGLT2-mediated glucose reabsorption, which is irrelevant to drug elimination. Option E is incorrect — glomerular filtration is pH-independent and only filters the unbound (free) drug fraction, not protein-bound drug

ANSWER: B

Rationale:

At steady state, the rate of drug input (infusion rate, R) equals the rate of drug elimination (CL × Css), such that plasma concentration remains constant: Css = R / CL. The time required to reach steady state is determined entirely by the drug's elimination half-life, regardless of the infusion rate — approximately 4 half-lives achieve ~94% of Css and approximately 5 half-lives achieve ~97%. This principle has two critical clinical implications: (1) doubling the infusion rate doubles Css but does not change the time to reach it; (2) a loading dose can be used to immediately achieve target Css when the half-life is long and waiting 4–5 half-lives is clinically unacceptable (e.g., digoxin, amiodarone, phenytoin). Option A is incorrect — Css is not defined by cumulative dose vs Vd; that relationship governs loading dose calculations. Option C is incorrect — Css is directly proportional to infusion rate (Css = R/CL); increasing the infusion rate increases Css. Option D is incorrect — large Vd prolongs the half-life and therefore delays the time to reach Css; it does not accelerate it. Option E is incorrect — Css is not a peak concentration; it is a plateau concentration achieved after multiple half-lives of accumulation.